The creature is seen in a video, viewed by more than 12,000 people on YouTube so far, clambering over the flood defences surrounding the RSPB's Fen Drayton Lakes reserve, near St Ives. An accompanying caption says: "The fields around the lakes have flooded, as they annually do. In the river is a seal that is climbing up the flood defences to get into the field, now a mere, with swollen water from the lake inside."

It appears to have swum up the Great Ouse river from King's Lynn, in Norfolk. Wildlife workers said seals were sometimes spotted in the river but this one was 10 miles further inland than previous sightings.

Graham Elliot, of the RSPB, said: "It would be the first time this has happened to my knowledge, that one has made its way into the lake.

"The floods normally come from ground water rather than the river and so if there is one it must have worked its way up in a ditch or something like that.

"It would be tremendous if one is there. It would be extremely unusual and it would become an attraction to visitors."

• This article was amended on 2 January 2013. The original said the seal appeared to have swum up the river Ouse. This has been corrected